7 replies to this topic

[ChemEng16]

Hi,

 

 Hydrocarbons with carbon dioxide are coming out of my reactor. At the stream pressure, using the antoine equation i've found that carbon dioxide is above its boiling point whereas the hydrocarbons are not. I am making the assumption that some of the carbon dioxide will be dissolved in the liquid stream , in which case I have to remove it.

 

I know how to perform standard gas liquid separation , (flash, distilation , etc) but I don't think it should undergo phase equilibrium separation, since carbon dioxide is already gas.

 

I think I have to use a knock-out drum, but the equations i've found only correlate the maximum vapour velocity to the separator empirical constant (Kv) and the densities of the liquid and gas phases. How does this take into consideration the dissolved vapour? I am supposed to assume that the liquid density includes the dissolved component?

 

V_v=K_v ((ρ_L-ρ_V)/ρ_V)^1/2

 

Also how does this take into account the residence time in the separator, the degree of separation achieved and the actual size of the separator? Is there something I am missing?Should I be performing a flash calculation instead?

 

Thanks

 

 

*Edit: Out of the hydrocarbon product mixture , another product is gaseous, which further complicates things.

Edited by ChemEng16, 12 March 2016 - 12:01 PM.

[latexman]

The equilibrium calculations give you information on the composition of vapor and liquid phases leaving the flash tank.  This information can be used to calculate stream densities.  Then, the flash tank can be sized.  So, there is an order to these calculations.

[ChemEng16]

First of all, thank you very much for your reply.

 

I've done the dew point and bubble point calculations, and because of the presence of CO2 the equilibrium temperatures are unreasonably low, so I am at the moment neglecting the possibility of such separation.

 

Is it absolutely necessary for separation to occur at equilibrium?In this particular case it wouldn't make sense, at least to me...

 

I've considered using an absorption column but unfortunately one of the (desired )hydrocarbon products(ethene) has a low volatility as well, so it would also be absorbed and lost.

I am trying to find a method to separate the carbon dioxide and ethene, and then compress the gaseous ethene in the hydrocarbon product mixture so it dissolves. I've been working on this for days and maybe I am not thinking clearly

 

The "problematic" gaseous mixture is 80% CO₂ and 20% C₂H₈

Edited by ChemEng16, 12 March 2016 - 12:04 PM.

[latexman]

Ethylene (ethene) is not C₂H₈, it is C₂H₄.  Ethylene has a lower nBP than CO2 so it can cause low temperatures too.  What is your pressure?  What temperatures are you seeing?  What heat and cooling sources do you have?

[ChemEng16]

My bad, I apologize , I meant butene (C₄H₈).The conditions of the stream are 450 K and 23 bar. The mass balances are given in the attachment , but the main points are that the CO₂  mass fraction is approximately half of the stream and that of butene is virtually negligible.

 

i have decided to compress the stream so that butene liquefies and then send it off to a stripping column to perform a liquid liquid extraction.  I am considering of using water, but havent seen many such examples , perhaps , not environmentally friendly? Nonetheless, I am thinking of doing that, and then sending the extract to a separator.

Attached Files

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Edited by ChemEng16, 12 March 2016 - 11:19 PM.

[breizh]

Hi,

You should use the search engine in this forum and check for CO2 capture or removal . A lot of documents are there about separation Co2 from natural gas using absorption with Amines ,hope this should put you on the right path .

 

Good luck

 

Breizh

Edited by breizh, 12 March 2016 - 11:53 PM.

[ChemEng16]

Thank you ,

 

I thought that amine treatment was used for absorption rather than extraction , or it doesn't matter?

[ChemEng16]

Dear breizh,

 

I had a go at the site's database , and i got my facts straight ! thanks for that,  i'm amazed at the community the site has going on.

 

If I may I want to ask another question, I am aware that MEA  has a very high affinity for absorbing carbon dioxide, but  isn't that at the risk of having some of the hydrocarbons getting dissolved/extracted as well? Is there some way to inhibit this that possibly I'm not aware  of? My initial guess to use water was primarily based on the fact that the hydrocarbon solubilityis significantly lower to almost negligible.

Edited by ChemEng16, 13 March 2016 - 07:01 AM.